Saturday, December 17, 2011

Einstein’s theory of general relativity : The interactions of mass (as related to gravitational force) are really the inﬂuence of bodies (masses) on the geometry of space and time.
Space and time are affected by gravity.

This theory is grounded on two main ideas: that the speed of light is a universal constant in all frames of reference and that gravitational ﬁelds are equivalent to acceleration for all frames of reference within the space-time continuum.

The ﬁrst proofs of the general theory of relativity came from astronomy. It explained the previously unknown reason for the variations in the motions of the planets. The theory then was used to predict the bending of starlight as it passed massive bodies, such as the sun, and as it was detected during a total eclipse.

The theory also predicted that electromagnetic radiation in a strong gravitational ﬁeld would shift the radiation to longer wavelengths. This was demonstrated by using the Mössbauer effect, which predicts the effects of a strong gravitational ﬁeld on radiation. An experiment using a strong source of gamma radiation was set up just seventy ﬁve feet above Earth to measure the gamma rays as they approached the surface of Earth. A minute lengthening of gamma rays (very short wavelength electromagnetic radiation) caused by the gravity of Earth was detected, thus conﬁrming the theory.

Einstein’s theory of photoelectric effect

Photoelectric effect :

The process in which visible light, x rays, or gamma rays incident on matter cause an electron to be ejected. The ejected electron is called a photoelectron.

In 1907 Einstein was awarded the Nobel Prize in physics for his explanation of the photoelectric effect.

Einstein’s explanation of the photoelectric effect was very simple. He assumed that the kinetic energy of the ejected electron was equal to the energy of the incident photon minus the energy required to remove the electron from the material, which is called the work function. Thus the photon hits a surface, gives nearly all its energy to an electron and the electron is ejected with that energy less whatever energy is required to get it out of the atom and away from the surface.

The energy of a photon is given by :

E = hg = hc/l where g is the frequency of the photon, l is

the wavelength, and c is the velocity of light.

This applies not only to light but also to x rays and gamma rays. Thus the shorter the wavelength the more energetic the photon.

Many of the properties of light such as interference

and diffraction can be explained most naturally by a

wave theory while others, like the photoelectric effect,

can only be explained by a particle theory. This peculiar

fact is often referred to as wave-particle duality and can

only be understood using quantum theory which must be

used to explain what happens on an atomic scale and

which provides a unified description of both processes.

Some applications of the photoelectric effect :

The photoelectric effect has many practical applications which include the photocell, photoconductive devices and solar cells. A photocell is usually a vacuum tube with two electrodes. One is a photosensitive cathode which emits electrons when exposed to light and the other is an anode which is maintained at a positive voltage with respect to the cathode. Thus when light shines on the cathode, electrons are attracted to the anode and an electron current flows in the tube from cathode to anode. The current can be used to operate a relay, which might turn a motor on to open a door or ring a bell in an alarm system.